In engines with multi-point injection, the injectors inject fuel into individual intake tubes, directly in front of the engine intake valves.
The injection valve, or nozzle valve, is held against the valve seat by a helical compression spring. A moveable plunger is rigidly attached to the nozzle, and supported in a guide in the lower section of the injector body. The plunger is acted on by a solenoid winding, wound onto the injector body. Each end connects to a terminal. The control unit completes the injector electrical circuit.
It provides pulses of a set duration, so that the injector valve opens and closes, or pulses, very quickly. Electrical pulses pass through the injector winding, and set up a magnetic field that draws the plunger and valve away from the nozzle seat. Fuel held under pressure in the fuel rail can now pass through the filter, and the center of the injector, and enter the inlet port.
In a single-hole injector, fuel is injected in a finely atomized, conical spray, at an angle of about 25°.
The exact injector open time depends on the data the sensors give the ECU, but it usually varies from 1 to 20 milliseconds. Valve-lift for the fully open position is about 0.15mm, and response time is about 1 millisecond.
Rubber moldings or ‘O’ rings seal the injector in the manifold and at the fuel rail. This helps prevent the transfer of heat to the injector, and the formation of fuel vapor bubbles under heat soak conditions.
Director-plate injectors operate on the same solenoid principle. 4 minute holes for fuel delivery direct jets of fuel toward each other, so that they collide and break up into a cone-shaped spray.
In some engines with 2 intake valves, the injectors have 4 holes but only 2 jets directed towards each valve. This ensures good fuel distribution for efficient combustion and emission control. The jets are set very precisely, and they must be located accurately by means of the fuel rail.
In side-feed injectors, the injector is located integrally with the fuel rail and fuel flows through the side of the injector near the bottom of the injector body. Fuel vapor is easily carried away by the return fuel, which provides easier starting when the engine is hot. Resin-molded connectors are used to reduce noise, and to retain and locate the injectors in the fuel rail.
All injectors are calibrated so that, in any application, the nozzle valve-lift, and the annular gap between the nozzle valve and the injector body, is the same for each injector.
Fuel pressure is held constant, so the only quantity that can change is how long the nozzle valve is held open, and this is what determines how much fuel is delivered. This is called the pulse width of the injector and it’s decided by the control unit.
The injectors fitted must have the correct calibration. Color coding of the terminal adaptor can be used for identification.
The injector used for throttle-body injection is similar to that used for multi-point. It has a nozzle and spring-loaded plunger, operated by a solenoid. When the injector opens, fuel is sprayed into the intake air passing through the throttle body.
There may be 1 or 2 injectors which open and close at a frequency set by the control unit. At low engine RPM, this frequency is low. As engine speed rises, so does frequency, until at some point, the valve may not close completely, and it then may deliver an almost continuous spray of fuel into the intake air entering the manifold.
In some cases, a cold-start injector is fitted. It is a solenoid-operated valve, on the intake manifold or plenum chamber. It is in the main airstream, on the engine side of the throttle butterfly, and it’s supplied with fuel under pressure from the fuel rail. A turbulence nozzle injects the atomized fuel into the airstream during engine cranking. The operation is controlled by the action of a thermotime switch.